In color image-forming devices such as color digital copy machines, generally, an image of a color original, inputted through a scanner, undergoes predetermined image processing to color separate and color convert the image into image data for each color, and a color printer section, which reproduces the image by image data for each color, outputs an image of the color original.
In addition, recently color digital copy machines have appeared on the market which, for high-speed reproduction of high-resolution color originals, include an image-forming device which uses laser recording scanning. In color digital copy machines of this type, by means of a laser deflection scanning section provided in each laser recording section, laser light modulated in accordance with digital image signals is guided to and performs scanning of a photoreceptor. The chief members of this laser deflection scanning unit are a rotating polygon mirror (hereinafter referred to as the "polygon mirror") and a motor which rotates the polygon mirror at high speed.
Further, recently color digital copy machines have appeared on the market which, in order to improve repeatability of black and of the color image as a whole, are typically provided with a Bk (black) recording section in addition to Y (yellow), M (magenta), and C (cyan) recording sections. For this reason, a monochrome mode is provided in addition to color mode, and thus both color and monochrome copying are possible.
In this kind of color digital copy machine, monochrome copying is set to be performed faster than color copying. This is because, in the case of, for example, the multiple revolution process method, which uses a single photoreceptor, color copying is a four-color process and monochrome copying a one-color process, and thus monochrome copying is necessarily faster. It is also due to the much greater frequency of monochrome over color copying.
In the tandem method, which uses a plurality of photoreceptors, unlike the multiple revolution process method, color and monochrome copying are performed at the same speed, but in order to respond to the demand for faster monochrome than color copying, process speeds such as surface speed of the photoreceptor, transport speed of the recording paper, etc., and the operating speed of the laser recording section which includes the laser scanning unit for black, are set faster than in the color mode.
In this case, the rotation speed of the motor which rotates the polygon mirror of the laser scanning unit of the laser recording section for black is switched between two speeds, one for monochrome mode and one for color mode. Further, if, during waiting, the motor is rotated at a waiting speed slower than that during color mode copying, the motor must be controlled by switching among three speeds.
In contrast, Japanese Unexamined Patent Publication No. 4-247418/1992 (Tokukaihei 4-247418) discloses a structure, in a monochrome laser printer, for reducing scanning density by skipping certain surfaces of the polygon mirror.
However, with the foregoing color digital copy machines structured so as to switch the speed of the motor of the laser recording section, a problem arises due to switching the speed of the motor which rotates the polygon mirror. When switching between monochrome and color modes (especially from monochrome to color), it takes a long time to obtain the first copy.
In other words, in switching the motor speed of the laser recording section for black, when monochrome mode is switched to color mode, it is necessary to bring the motor speed of the laser recording section for black into conformity with the motor speed of the laser recording sections for color, and to synchronize their mirror surfaces, but it takes a long time to attain this synchronization.
This is because it is difficult to stabilize the motor speed of the polygon mirror when its speed is reduced from the high-speed monochrome mode to the low-speed color mode, and this stabilization requires a great amount of time. As a result, the start of copying immediately after switching the copy mode is delayed, and the time required to obtain the first copy is lengthened. Incidentally, with conventional technology, motor speed stabilizes relatively quickly when motor speed is increased.